Changes in network activity with the progression of Parkinson's disease

Abnormalities in metabolic network activity precede the onset of motor symptoms in Parkinson's disease

Imaging studies show that Parkinson's disease (PD) alters the activity of motor- and cognition-related metabolic brain networks. However, it is not known whether the network changes appear at or before symptom onset. In this study, we examined 15 hemiparkinsonian patients who underwent serial metabolic imaging with [(18)F]-fluorodeoxyglucose (FDG) PET at baseline and again 2.1 +/- 0.6 (mean +/- SD) and 3.9 +/- 0.7 years later. We assessed longitudinal changes in network activity in each cerebral hemisphere, focusing specifically on the "presymptomatic" hemisphere--ipsilateral to the initially involved body side. At the network level, the activity of the PD motor-related pattern (PDRP) increased symmetrically in both hemispheres over time (p < 0.001), with significant bilateral elevations at each of the three time points. Hemispheric expression of the PD cognition-related pattern likewise increased symmetrically (p < 0.001), although significant elevations were not evident on either side until 4 years. At the regional level, putamen metabolism contralateral to the initially affected body side was elevated at all three time points, without longitudinal change. In contrast, in the initially presymptomatic hemisphere, putamen metabolic activity increased steadily over time, reaching abnormal levels only at 4 years. Metabolic activity in the contralateral precuneus fell to subnormal levels by the final time point. These findings suggest that abnormal PDRP activity antecedes the appearance of motor signs by approximately 2 years. The timing and laterality of symptom onset relates to focal asymmetric metabolic changes at the putamenal node of this network.

Metabolic networks for assessment of therapy and diagnosis in Parkinson's disease

Neuroimaging and modern computational techniques like spatial covariance analysis have contributed greatly to the understanding of neural system abnormalities in neurodegenerative disorders such as Parkinson's disease (PD). The application of network analysis to metabolic PET data obtained from patients with PD has led to the identification and validation of two distinct spatial covariance patterns associated with the motor and cognitive manifestations of the disease. Quantifying the activity of these patterns in individual subjects has provided an objective tool for the assessment of treatment efficacy and differential diagnosis. We have found that activity of the PD motor-related network is modulated by antiparkinsonian treatments such as dopaminergic therapy, deep brain stimulation (DBS), and subthalamic nucleus (STN) gene therapy. By contrast, the cognitive-related network is not altered by these interventions for PD motor symptoms. This pattern may however change in response to therapies targeting the cognitive symptoms of this disorder. Recent work has focused on the identification of specific network biomarkers for atypical parkinsonian conditions such as multiple system atrophy (MSA) and progressive supranuclear palsy (PSP). These disease-related patterns can potentially be used in an automated imaging-based algorithm to classify patients with these disorders.

Differential diagnosis of parkinsonism: a metabolic imaging study using pattern analysis

BACKGROUND

Idiopathic Parkinson's disease can present with symptoms similar to those of multiple system atrophy or progressive supranuclear palsy. We aimed to assess whether metabolic brain imaging combined with spatial covariance analysis could accurately discriminate patients with parkinsonism who had different underlying disorders.

METHODS

Between January, 1998, and December, 2006, patients from the New York area who had parkinsonian features but uncertain clinical diagnosis had fluorine-18-labelled-fluorodeoxyglucose-PET at The Feinstein Institute for Medical Research. We developed an automated image-based classification procedure to differentiate individual patients with idiopathic Parkinson's disease, multiple system atrophy, and progressive supranuclear palsy. For each patient, the likelihood of having each of the three diseases was calculated by use of multiple disease-related patterns with logistic regression and leave-one-out cross-validation. Each patient was classified according to criteria defined by receiver-operating-characteristic analysis. After imaging, patients were assessed by blinded movement disorders specialists for a mean of 2.6 years before a final clinical diagnosis was made. The accuracy of the initial image-based classification was assessed by comparison with the final clinical diagnosis.

FINDINGS

167 patients were assessed. Image-based classification for idiopathic Parkinson's disease had 84% sensitivity, 97% specificity, 98% positive predictive value (PPV), and 82% negative predictive value (NPV). Imaging classifications were also accurate for multiple system atrophy (85% sensitivity, 96% specificity, 97% PPV, and 83% NPV) and progressive supranuclear palsy (88% sensitivity, 94% specificity, 91% PPV, and 92% NPV).

INTERPRETATION

Automated image-based classification has high specificity in distinguishing between parkinsonian disorders and could help in selecting treatment for early-stage patients and identifying participants for clinical trials.

FUNDING

National Institutes of Health and General Clinical Research Center at The Feinstein Institute for Medical Research.

Imaging the nigrostriatal system to monitor disease progression and treatment-induced complications

Radiotracer imaging (RTI) techniques such as positron emission tomography (PET) allow the in vivo assessment of nigrostriatal DA function in Parkinson's disease and have provided valuable insights into the mechanisms of nigrostriatal degeneration and the consequent compensatory changes. Moreover, functional imaging serves as an excellent tool in the assessment of the progression of PD and the evolution of treatment-related complications. However, various studies have shown discordance between clinical progression of PD and nigrostriatal degeneration estimated by PET or SPECT, and no RTI technique can be reliably used as a biomarker for progression of PD. Presynaptic dopaminergic imaging has consistently demonstrated an anterior-posterior gradient of dopaminergic dysfunction predominantly affecting the putamen, with side-to-side asymmetry in tracer binding. Dopaminergic hypofunction in the striatum follows a negative exponential pattern with the fastest rate of decline in early disease. Evaluation of central pharmacokinetics of levodopa action by PET has demonstrated the role of increased synaptic dopamine turnover and downregulation of the dopamine transporter in the pathophysiology of levodopa-induced dyskinesias. In PD with behavioral complications such as impulse control disorders, increased levels of dopamine release have been observed in the ventral striatum during performance of a positive reward task, as well as loss of deactivation in orbitofrontal cortex in response to negative reward prediction errors. This suggests that there is a pathologically heightened "reward" response in the ventral striatum together with loss of the capacity to respond to negative outcomes. Overall, functional imaging with PET is an excellent tool for understanding the disease and its complications; however, caution must be applied in interpretation of the results.

FDG PET in the Evaluation of Parkinson's Disease

Network analysis of (18)F-fluorodeoxyglucose (FDG) positron emission tomography (PET) is an innovative approach for the study of in movement disorders, such as Parkinson's disease (PD). Spatial covariance analysis of imaging data acquired from PD patients has revealed characteristic regional patterns associated with the motor and cognitive features of disease. Quantification of pattern expression in individual patients can be used for diagnosis, assessment of disease severity, and evaluation of novel medical and surgical therapies. Identification of disease-specific patterns in other parkinsonian syndromes, such as multiple system atrophy and progressive supranuclear palsy, has improved diagnostic accuracy in patients with difficult to diagnose parkinsonism. Further developments of these techniques are likely to enhance the role of functional imaging in investigating underlying abnormalities and potential new therapies in these neurodegenerative diseases.

Dynamic changes in cerebello-thalamo-cortical motor circuitry during progression of Parkinson's disease

Both the basal ganglia and cerebellum are known to influence cortical motor and motor-associated areas via the thalamus. Whereas striato-thalamo-cortical (STC) motor circuit dysfunction has been implicated clearly in Parkinson's disease (PD), the role of the cerebello-thalamo-cortical (CTC) motor circuit has not been well defined. Functional magnetic resonance imaging (fMRI) is a convenient tool for studying the role of the CTC in vivo in PD patients, but large inter-individual differences in fMRI activation patterns require very large numbers of subjects in order to interpret data from cross-sectional, case control studies. To understand the role of the CTC during PD progression, we obtained longitudinal fMRI 2 years apart from 5 PD (57+/-8 yr) and five Controls (57+/-9 yr) performing either externally- (EG) or internally-guided (IG) sequential finger movements. All PD subjects had unilateral motor symptoms at baseline, but developed bilateral symptoms at follow-up. Within-group analyses were performed by comparing fMRI activation patterns between baseline and follow-up scans. Between-group comparisons were made by contrasting fMRI activation patterns generated by the more-affected and less-affected hands of PD subjects with the mean of the dominant and non-dominant hands of Controls. Compared to baseline, Controls showed changes in CTC circuits, but PD subjects had increased recruitment of both cortical motor-associated and cerebellar areas. Compared to Controls, PD subjects demonstrated augmented recruitment of CTC circuits over time that was statistically significant when the IG task was performed by the hand that transitioned from non-symptomatic to symptomatic. This longitudinal fMRI study demonstrates increased recruitment of the CTC motor circuit concomitant with PD progression, suggesting a role of the CTC circuit in accommodation to, or pathophysiology of, PD.

Dopamine cell implantation in Parkinson's disease: long-term clinical and (18)F-FDOPA PET outcomes

We have previously reported the results of a 1-y double-blind, placebo-controlled study of embryonic dopamine cell implantation for Parkinson's disease. At the end of the blinded phase, we found a significant increase in putamen uptake on (18)F-fluorodopa ((18)F-FDOPA) PET reflecting the viability of the grafts. Nonetheless, clinical improvement was significant only in younger (age < or = 60 y) transplant recipients, as indicated by a reduction in Unified Parkinson's Disease Rating Scale (UPDRS) motor scores.

METHODS

We now report long-term clinical and PET outcomes from 33 of the original trial participants who were followed for 2 y after transplantation and 15 of these subjects who were followed for 2 additional years. Longitudinal changes in UPDRS motor ratings and caudate and putamen (18)F-FDOPA uptake were assessed with repeated-measures ANOVA. Relationships between these changes over time were evaluated by the analysis of within-subject correlations.

RESULTS

We found that UPDRS motor ratings declined over time after transplantation (P < 0.001). Clinical improvement at 1 y was relatively better for the younger transplant recipients and for men, but these age and sex differences were not evident at longer-term follow-up. Significant increases in putamen (18)F-FDOPA uptake were evident at all posttransplantation time points (P < 0.001) and were not influenced by either age or sex. Posttransplantation changes in putamen PET signal and clinical outcome were significantly intercorrelated (P < 0.02) over the course of the study. Image analysis at the voxel level revealed significant bilateral increases in (18)F-FDOPA uptake at 1 y (P < 0.001) in the posterior putamen engraftment sites. PET signal in this region increased further at 2 and 4 y after engraftment. Concurrently, this analysis disclosed progressive declines in radiotracer uptake in the nonengrafted caudate and ventrorostral putamen. Clinical improvement after transplantation correlated with the retention of PET signal in this region at the preoperative baseline.

CONCLUSION

These results suggest that clinical benefit and graft viability are sustained up to 4 y after transplantation. Moreover, the dependence of clinical (but not imaging) outcomes on subject age and sex at 1 y may not persist over the long term. Last, the imaging changes reliably correlate with clinical outcome over the entire posttransplantation time course.

Progression of Parkinson's disease in the clinical phase: potential markers

Neuromodulatory or even neuroprotective therapy could soon be available for Parkinson's disease (PD), raising the question of how we should define and measure disease progression. Reported evidence suggests that several symptoms worsen with disease duration. Bradykinesia, rigidity, and activities of daily living deteriorate faster at the beginning of the disease, and this deterioration is paralleled by a decline in functional presynaptic dopaminergic activity, as shown by imaging techniques. Cognitive, speech, sleep, and gait difficulties might progress linearly in proportion to disease duration. Reduced variability in heart rate, orthostatic dysfunction, and visual hallucinations start to develop at mid-stage disease and are more common in late stages than earlier stages. In this Review, we summarise our current understanding of the progression of PD-associated symptoms and markers and conclude that an effective measurement of progression of PD must adapt to the different stages of the disease. In addition to routine clinical rating scales, new quantitative assessments of motor and non-motor symptoms, which should be more broadly available, reasonably priced, and easy-to-use, are needed.

Early Parkinson's disease: longitudinal changes in brain activity during sequence learning

Impairment of sequence learning is common in Parkinson's disease (PD), but the time course of this cognitive abnormality is not known. We assessed longitudinal changes in sequence learning performance and associated task-related cerebral blood flow in 13 early stage PD patients who underwent H(2)(15)O PET at baseline and again 2 years later. Ten healthy volunteer subjects served as controls. A trend toward decline in learning performance (p=0.08) was evident over the 2 years of follow-up. During this interval, significant declines in learning-related activation were detected in parietal and temporo-occipital association areas and in the right dorsolateral prefrontal cortex. Learning-related activation in these regions was normal at baseline, but declined to subnormal levels (p<0.01) at 2 years. Significant hippocampal activation (p<0.005) was present in the subjects with high learning performance over time. The findings are consistent with a decline in learning-related neural activity in cortical areas with prominent Lewy body formation.

PET functional imaging of deep brain stimulation in movement disorders and psychiatry

Deep brain stimulation (DBS) represents a major advance in the treatment of various severe movement disorders or neuropsychiatric diseases. Our understanding of the mechanism of action of this surgical treatment has greatly benefited from functional imaging studies. Most of these studies have been conducted in patients with Parkinson's disease (PD) treated by bilateral subthalamic nucleus (STN) stimulation. These studies have notably underlined the fact that STN stimulation influences motor, limbic, or associative cortical-subcortical loops in various (sometimes contradictory) ways. We present an up-to-date review of the information provided by functional imaging studies in surgery for PD, dystonia, tremor, as well as in psychiatric disorders such as depression or obsessive-compulsive disorder. On the basis of this information, proposed mechanisms of action of DBS are discussed, as well as the need for additional approaches such as improved anatomical localization of the contact used for stimulation or a better understanding of the electrical distribution around the electrode.

Metabolic brain networks in neurodegenerative disorders: a functional imaging approach

Network analysis of functional brain imaging data is an innovative approach to study circuit abnormalities in neurodegenerative diseases. In Parkinson's disease, spatial covariance analysis of resting-state metabolic images has identified specific regional patterns associated with motor and cognitive symptoms. With functional imaging, these metabolic networks have recently been used to measure system-related progression and to evaluate novel treatment strategies. Network analysis is also being used to characterize specific functional biomarkers for Huntington's disease and Alzheimer's disease. These networks have been particularly helpful in uncovering compensatory mechanisms in genetically at-risk individuals. Ongoing developments in network applications are likely to enhance the role of functional imaging in the investigation of neurodegenerative disorders.

Cognitive impairment in Parkinson's disease: tools for diagnosis and assessment

Cognitive impairment (CI) and dementia are frequent and debilitating features associated with Parkinson's disease (PD). Formal neuropsychological examination is required to ascertain the degree and pattern of CI over the course of the disease. The use of different tools may explain heterogeneous data obtained from studies to date. Normative data for extensively used scales [Mattis Dementia Rating Scale (MDRS), Mini-Mental State Examination (MMSE)] is incomplete in PD populations. According to sample characteristics, statistical analyses, and methodological quality, 33 studies using scales not specific to PD (MDRS, MMSE, Cambridge Cognitive Assessment, FAB) or PD-specific scales (Mini-Mental Parkinson, Scales for Outcomes of Parkinson's disease-Cognition, Parkinson's Disease-Cognitive Rating Scale, and Parkinson Neuropsychometric Dementia Assessment) were eligible for the critical analysis of their appropriateness to assess cognition in PD. Of the four scales specifically designed for PD, the SCOPA-COG and the PD-CRS have undergone extensive and rigorous validation processes. While the SCOPA-COG mainly assesses "frontal-subcortical" cognitive defects, the PD-CRS also assesses "instrumental-cortical" functions, allowing better characterization of the different patterns of CI that may be present in PD from the earliest stages. The MMP and PANDA scales were designed as brief screening tests for CI and have not yet been subjected to extensive clinimetric evaluations. Further research on PD-specific tools seems mandatory to help establish accurate cut-off scores for the diagnosis of mild PDD, detect cognitive profiles more prone to the future development of dementia, and allow comparisons between different descriptive or interventional studies.

Effect of prenatal manganese intoxication on [(3)H]glucose uptake in the brain of rats lesioned as neonates with 6-hydroxydopamine

In the present study we examined the effects of prenatal manganese (Mn) intoxication on [(3)H]glucose uptake in the brain of rats lesioned as neonates with 6-hydroxydopamine (6-OHDA). MnCl(2) . 4H(2)O (10,000 ppm) was added to the drinking water of pregnant Wistar rats for the duration of pregnancy. On the day of parturition, Mn was discontinued as an additive to the drinking water. The control group consisted of rats that consumed water without Mn. Three days after birth, rats in both groups (control and Mn) were pretreated with desipramine hydrochloride (20 mg/kg) and pargyline hydrochloride (50 mg/kg) and injected bilaterally icv with one of three doses of 6-OHDA hydrobromide (15 mug, 30 mug or 67 mug base form in saline on each side) or with saline (control). 6-[(3)H]-D-glucose (500 muCi/kg, ip) was administered to male offspring in adulthood; after 15 min, brain specimens were taken (frontal cortex, hippocampus, striatum, thalamus with hypothalamus, pons and cerebellum) for determination of radioactivity in a liquid scintillation counter. Low dose 6-OHDA (15 mug icv) increased [(3)H]glucose uptake in all brain regions (p < 0.05) in both control and Mn-intoxicated animals. In rats lesioned with a moderate dose of 6-OHDA (30 mug icv), [(3)H]glucose uptake was unaltered in both control and Mn-exposed rats. High dose 6-OHDA (67 mug icv) reduced [(3)H]glucose uptake in all brain regions of Mn-exposed rats (except for cerebellum) compared with the saline group (all, p < 0.05). There was no change in regional brain uptake of [(3)H]glucose in control rats. In conclusion, this study shows that mild neuronal insult (15 mug icv 6-OHDA) increased glucose uptake in the brain while severe damage (concomitant 60 mug icv 6-OHDA and Mn treatment) significantly diminished this process.

Subcortical elevation of metabolism in Parkinson's disease--a critical reappraisal in the context of global mean normalization

In a recent issue of NeuroImage, we presented evidence that biased global mean (GM) normalization of brain PET data can generate the appearance of subcortical foci with relative hypermetabolism in patients with Parkinson's disease (PD), and other degenerative disorders. In a commentary to our article, Ma and colleagues presented a study seeking to establish that a pattern of widespread hypermetabolism, known as the Parkinson's disease related pattern (PDRP) is a genuine metabolic feature of PD. In the present paper, we respond to the arguments presented by Ma et al., and we provide a critical reappraisal of the evidence for the existence of the PDRP. To this end, we present new analyses of PET data sets, which demonstrate that very similar patterns of relative subcortical increases are seen in PD, Alzheimer's disease, hepatic encephalopathy, healthy aging, and simulation data. Furthermore, longitudinal studies of PD previously reported relative hypermetabolism in very small anatomical structures such as the subthalamic nucleus. We now demonstrate how focal hypermetabolism attributed to small nuclei can similarly arise as a consequence of GM normalization. Finally, we give a comprehensive summary of the entire deoxyglucose autoradiography literature on acquired parkinsonism in experimental animals. Based on this evidence, we conclude that (1) there is no quantitative evidence for widespread subcortical hypermetabolism in PD, (2) very similar patterns of subcortical hyperactivity are evident in various other brain disorders whenever GM normalization is utilized, and (3) the PDRP is not evident in animal models of PD. In the absence of quantitative evidence for the PDRP, our alternative interpretation of normalization bias seems the more parsimonious explanation for the reports of relative hypermetabolism in PD.

Increased oxidation of certain glycolysis and energy metabolism enzymes in the frontal cortex in Lewy body diseases

Lipoxidative damage of aldolase A, enolase 1, and glyceraldehyde dehydrogenase (GAPDH) was found in the frontal cortex in a percentage of aged controls by bidimensional gel electrophoresis, Western blot test, in-gel digestion, and mass spectrometry. Aldolase A and enolase 1 were altered in 12 of 19 cases, whereas oxidation of GAPDH was found in 6 of 19 controls. The three enzymes were oxidized in the frontal cortex in the majority of cases of incidental Parkinson's disease (iPD), PD, and dementia with Lewy bodies (DLB). Differences were statistically significant (chi(2) test) for GAPDH in PD and DLB. Densitometric studies have shown that the ratio of oxidized protein per spot is higher in iPD, PD, and DLB compared with controls. These findings show oxidation of three enzymes linked with glycolysis and energy metabolism in the adult human brain as well as increased oxidation of aldolase A, enolase 1, and GAPDH in the frontal cortex in Lewy body diseases. Modifications of these enzymes may result in decreased activity and may partly account for impaired metabolism and function of the frontal lobe in PD.

Single unit "pauser" characteristics of the globus pallidus pars externa distinguish primary dystonia from secondary dystonia and Parkinson's disease

The presence of high frequency discharge neurons with long periods of silence or "pauses" in the globus pallidus pars externa (GPe) is a unique identifying feature of this nucleus. Prior studies have demonstrated that pause characteristics reflect synaptic inputs into GPe. We hypothesized that GPe pause characteristics should distinguish movement disorders whose basal ganglia network abnormalities are different. We examined pause characteristics in 224 GPe units in patients with primary generalized dystonia, Parkinson's disease (PD), and secondary dystonia, undergoing single unit microelectrode recording for DBS placement in the awake state. Pauses in neuronal discharge were identified using the Poisson surprise method. Mean pause length in primary dystonia (606.8373.3) was higher than in PD (557.4366.6) (p<0.05). Interpause interval (IPI) was lower in primary dystonia (2331.63874.1) than PD (3646.45894.5) (p<0.01), and mean pause frequency was higher in primary dystonia (0.140.10) than PD (0.070.12) (p<0.01). Comparison of pause characteristics in primary versus secondary generalized dystonia revealed a significantly longer mean pause length in primary (606.8373.3) than in secondary dystonia (495.6236.5) (p<0.01). IPI was shorter in primary (2331.6+/-3874.1) than in secondary dystonia (3484.5+/-3981.6) (p<0.01). The results show that pause characteristics recorded in the awake human GPe distinguish primary dystonia from Parkinson's disease and secondary dystonia. The differences may reflect increased phasic input from striatal D2 receptor positive cells in primary dystonia, and are consistent with a recent model proposing that GPe provides capacity scaling for cortical input.

Abnormal regional brain function in Parkinson's disease: truth or fiction?

Normalization of regional measurements by the global mean is commonly employed to minimize inter-subject variability in functional imaging studies. This practice is based on the assumption that global values do not substantially differ between patient and control groups. In this issue of NeuroImage, Borghammer and colleagues challenge the validity of this assumption. They focus on Parkinson's disease (PD) and use computer simulations to show that lower global values can produce spurious increases in subcortical brain regions. The authors speculate that the increased signal observed in these areas in PD is artefactual and unrelated to localized changes in brain function. In this commentary, we summarize what is currently known of the relationship between regional and global metabolic activity in PD and experimental parkinsonism. We found that early stage PD patients exhibit global values that are virtually identical to those of age-matched healthy subjects. SPM analysis revealed increased normalized metabolic activity in a discrete set of biologically relevant subcortical brain regions. Because of their higher variability, the corresponding absolute regional measures did not differ across the two groups. Longitudinal imaging studies in this population showed that the subcortical elevations in normalized metabolism appeared earlier and progressed faster than did focal cortical or global metabolic reductions. The observed increases in subcortical activity, but not the global changes, correlated with independent clinical measures of disease progression. Multivariate analysis with SSM/PCA further confirmed that the abnormal spatial covariance structure of early PD is dominated by these subcortical increases as opposed to network-related reductions in cortical metabolic activity or global changes. Thus, increased subcortical activity in PD cannot be regarded as a simple artefact of global normalization. Moreover, stability of the normalized measurements, particularly at the network level, makes these metabolic indices suitable as imaging biomarkers of PD progression and the treatment response.

Data-driven intensity normalization of PET group comparison studies is superior to global mean normalization

BACKGROUND

Global mean (GM) normalization is one of the most commonly used methods of normalization in PET and SPECT group comparison studies of neurodegenerative disorders. It requires that no between-group GM difference is present, which may be strongly violated in neurodegenerative disorders. Importantly, such GM differences often elude detection due to the large intrinsic variance in absolute values of cerebral blood flow or glucose consumption. Alternative methods of normalization are needed for this type of data.

MATERIALS AND METHODS

Two types of simulation were performed using CBF images from 49 controls. Two homogeneous groups of 20 subjects were sampled repeatedly. In one group, cortical CBF was artificially decreased moderately (simulation I) or slightly (simulation II). The other group served as controls. Ratio normalization was performed using five reference regions: (1) Global mean; (2) An unbiased VOI; (3) Data-driven region extraction (Andersson); (4-5) Reference cluster methods (Yakushev et al.). Using voxel-based statistics, it was determined how much of the original signal was detected following each type of normalization.

RESULTS

For both simulations, global mean normalization performed poorly, with only a few percent of the original signal recovered. Global mean normalization moreover created artificial increases. In contrast, the data-driven reference cluster method detected 65-95% of the original signal.

CONCLUSION

In the present simulation, the reference cluster method was superior to GM normalization. We conclude that the reference cluster method will likely yield more accurate results in the study of patients with early to moderate stage neurodegenerative disorders.

Scientific rationale for the development of gene therapy strategies for Parkinson's disease

The ever-evolving understanding of the neuronal systems involved in Parkinson's disease together with the recent advances in recombinant viral vector technology has led to the development of several gene therapy applications that are now entering into clinical testing phase. To date, four fundamentally different approaches have been pursued utilizing recombinant adeno-associated virus and lentiviruses as vectors for delivery. These strategies aim either to restore the lost brain functions by substitution of enzymes critical for synthesis of neurotransmitters or neurotrophic factors as a means to boost the function of remaining neurons in the diseased brain. In this review we discuss the differences in mechanism of action and describe the scientific rationale behind the currently tested gene therapy approaches for Parkinson's disease in some detail and pinpoint their individual unique strengths and weaknesses.

Variability of EMG patterns: a potential neurophysiological marker of Parkinson's disease?

OBJECTIVE

This study evaluated whether changes in the electromygraphic (EMG) pattern during rapid point-to-point movements in individuals diagnosed with PD can: (1) distinguish PD subjects from healthy subjects and (2) determine if differences in the EMG pattern reflect disease severity in PD.

METHODS

Three groups of 10 PD subjects and 10 age/sex-matched healthy subjects performed rapid 72 degree point-to-point elbow flexion movements. PD subjects were divided, a priori, into three groups based upon off medication motor UPDRS score.

RESULTS

Measures related to the EMG pattern distinguished all PD subjects and 9 out of 10 healthy subjects, resulting in 100% sensitivity. Further, significant correlations were shown between EMG measures and the motor UPDRS score. After 30 months, the one healthy subject whose EMG pattern was abnormal was reexamined. The EMG measures remained abnormal and the motor UPDRS score went from 0 to 10. Parkinson's disease was diagnosed.

CONCLUSION

Measures related to the variability of the EMG pattern during rapid point-to-point movements provide neurophysiological measures that objectively distinguish PD subjects from healthy subjects. These measures also correlate with disease severity.

SIGNIFICANCE

EMG measures may provide a non-invasive measure that is sensitive and specific for identifying individuals with PD.

Systems biology and its application to the understanding of neurological diseases

Recent advances in molecular biology, neurobiology, genetics, and imaging have demonstrated important insights about the nature of neurological diseases. However, a comprehensive understanding of their pathogenesis is still lacking. Although reductionism has been successful in enumerating and characterizing the components of most living organisms, it has failed to generate knowledge on how these components interact in complex arrangements to allow and sustain two of the most fundamental properties of the organism as a whole: its fitness, also termed its robustness, and its capacity to evolve. Systems biology complements the classic reductionist approaches in the biomedical sciences by enabling integration of available molecular, physiological, and clinical information in the context of a quantitative framework typically used by engineers. Systems biology employs tools developed in physics and mathematics such as nonlinear dynamics, control theory, and modeling of dynamic systems. The main goal of a systems approach to biology is to solve questions related to the complexity of living systems such as the brain, which cannot be reconciled solely with the currently available tools of molecular biology and genomics. As an example of the utility of this systems biological approach, network-based analyses of genes involved in hereditary ataxias have demonstrated a set of pathways related to RNA splicing, a novel pathogenic mechanism for these diseases. Network-based analysis is also challenging the current nosology of neurological diseases. This new knowledge will contribute to the development of patient-specific therapeutic approaches, bringing the paradigm of personalized medicine one step closer to reality.

Differential diagnosis of parkinsonian syndromes using PCA-based functional imaging features

In the current paper, we describe methodologies for single subject differential diagnosis of degenerative brain disorders using multivariate principal component analysis (PCA) of functional imaging scans. An automated routine utilizing these methods is applied to positron emission tomography (PET) brain data to distinguish several discrete parkinsonian movement disorders with similar clinical manifestations. Disease specific expressions of voxel-based spatial covariance patterns are predetermined using the Scaled Subprofile Model (SSM/PCA) and a scalar measure of the manifestation of each pattern in prospective subject images is subsequently derived. Scores are automatically compared to reference values generated for each pathological condition in a corresponding set of patient and control scans. Diagnostic outcome is optimized using strategies such as the derivation of patterns in a voxel subspace that reflects contrasting image characteristics between conditions, or by using an independent patient population as controls. The prediction models for two, three and four way classification problems using direct scalar comparison as well as classical discriminant analysis are assessed in a composite training population comprised of three different patient classes and normal controls, and validated in a similar independent test population. Results illustrate that highly accurate diagnosis can often be achieved by simple comparison of scores utilizing optimized patterns.

Brain imaging in patients with freezing of gait

Freezing of gait (FOG) is a disabling gait disturbance with unknown cerebral pathophysiology. In this review, we discuss the functional brain imaging studies that address gait physiology and pathophysiology of FOG. Radiotracer metabolic studies show basal ganglia-cortical circuitry involvement in different aspects of gait control. FOG patients showed orbitofrontal and posterior parietal deficits and possibly predominant involvement of right-sided circuitry. We suggest that FOG results from neuronal circuitry dysfunction in right-sided parietal-lateral premotor circuits. These circuits incorporate sensory information into the control of gait. Furthermore, abnormal function of frontostriatal loops, which probably sheer cognitive and attentional activities is also a main factor in FOG. Gait-induced brain circuitry activation can not adequately be achieved when investigated subjects are in a supine rest position, as is the case in most present day imaging studies. Some radiotracer activation studies were performed after walking. Imagination of gait has been used in some radiotracer activation studies with positron emission tomography (PET) and in studies with functional magnetic resonance imaging (fMRI), showing cortical activation patterns involved in several aspects of gait control. For future investigation of FOG, it is suggested to design PET and fMRI studies which concentrate on activation of neuropsychological and sensory integration circuits.

Positron emission tomography for neurologists

This short review focuses on practical, present day, clinical application of FDG PET, a technology available to practicing neurologists for managing their patients. Indications in the disease states of dementia, neuro-oncology, epilepsy, parkinsonism, and other less common settings are reviewed. Many third-party payers currently make reimbursements based on these indications. By measuring an aspect of brain function, PET provides information that often is unobtainable from other sources, thus facilitating more rationale and cost-effective management, which can only benefit the patient, the referring physician, and the health care system as a whole.

Cortical functional anatomy of voluntary saccades in Parkinson disease

In Parkinson Disease (PD) several aspects of saccades are affected. The saccade-generating brainstem neurons are spared, however, the signals they receive may be flawed. In particular voluntary saccades suffer, but the functional anatomy of the impairment of saccade-related cortical control is unknown. We measured blood-oxygenation-level-dependent (BOLD) activation with functional Magnetic Resonance Imaging (fMRI) while healthy participants and patients with PD performed horizontal voluntary saccades between peripheral visual targets or fixated centrally. We compared saccade-related BOLD-activity vs. fixation in patients with PD and in healthy controls and correlated perisaccadic BOLD-activity in PD patients with saccade kinetics (multistep saccades). Saccade related BOLD-activation was found in both PD and healthy participants in the superior parietal cortex (PEF) and the occipital cortex. Our results suggest remarkable hypoactivity of the frontal and supplementary eye fields (FEF and SEF) in PD patients. On the other hand, PD patients showed a statistically more reliable BOLD modulation than healthy participants in the posterior cingulate gyrus, the parahippocampal gyrus, inferior parietal lobule, precuneus and in the middle temporal gyrus. Given abnormal frontal and normal PEF responses, our results suggest that in PD a frontal cortical circuitry, known to be associated with saccade planning, selection, and predicting a metric error of the saccade, is deficient.

FDG-PET findings in patients with galactosaemia

Despite treatment with a galactose-restricted diet, many galactosaemia patients develop lifelong cognitive impairment, speech abnormalities and a gamut of neurological problems including cognitive impairment and tremors. No study has explored changes in cerebral glucose metabolism in patients with galactosaemia. Five patients with galactosaemia had ages ranging from 20 to 40 years (mean age 28 years) and eight similarly aged controls received brain [(18)F]fluorodeoxyglucose (FDG) positron emission tomography (PET) scans. PET scans were analysed using a previously validated template methodology of regions of interest (ROIs). Count ratios for each anatomical ROI were compared between the galactosaemic patients and the healthy controls. Statistical parametric mapping (SPM) software was also used to further analyse the data. ROI analysis showed that galactosaemic patients had significant bilateral decreases in cerebral glucose metabolism in the superior temporal gyrus, medial occipital lobe, parietal lobe, cerebellum, calcarine cortex, superior frontal cortex, and superior parietal cortex when compared with controls. Significant increases were seen in the cingulate gyrus and temporal poles, bilaterally. SPM analysis revealed foci of decreased glucose metabolism in the caudate, cerebellum, precentral gyrus and cerebellar tonsils of galactosaemic patients. SPM also showed increased glucose metabolism in the subcallosal gyrus and claustrum. The results show significant abnormalities in cerebral function in patients with galactosaemia, particularly with widespread decreases in cortical metabolism. These abnormalities appear to be in brain regions that may be associated with the neuropsychological deficits in these patients. PET brain scans may be of value in galactosaemia patients to evaluate for dysfunction.

Subcortical processes of motor response inhibition during a stop signal task

Previous studies have delineated the neural processes of motor response inhibition during a stop signal task, with most reports focusing on the cortical mechanisms. A recent study highlighted the importance of subcortical processes during stop signal inhibition in 13 individuals and suggested that the subthalamic nucleus (STN) may play a role in blocking response execution (Aron and Poldrack, 2006. Cortical and subcortical contributions to Stop signal response inhibition: role of the subthalamic nucleus. J Neurosci 26, 2424-2433). Here in a functional magnetic resonance imaging (fMRI) study we replicated the finding of greater activation in the STN during stop (success or error) trials, compared to go trials, in a larger sample of subjects (n=30). However, since a contrast between stop and go trials involved processes that could be distinguished from response inhibition, the role of subthalamic activity during stop signal inhibition remained to be specified. To this end we followed an alternative strategy to isolate the neural correlates of response inhibition (Li et al., 2006a. Imaging response inhibition in a stop signal task--neural correlates independent of signal monitoring and post-response processing. J Neurosci 26, 186-192). We compared individuals with short and long stop signal reaction time (SSRT) as computed by the horse race model. The two groups of subjects did not differ in any other aspects of stop signal performance. We showed greater activity in the short than the long SSRT group in the caudate head during stop successes, as compared to stop errors. Caudate activity was positively correlated with medial prefrontal activity previously shown to mediate stop signal inhibition. Conversely, bilateral thalamic nuclei and other parts of the basal ganglia, including the STN, showed greater activation in subjects with long than short SSRT. Thus, fMRI delineated contrasting roles of the prefrontal-caudate and striato-thalamic activities in mediating motor response inhibition.

Striatal dopamine and working memory

Recent studies have emphasized the importance of dopamine projections to the prefrontal cortex (PFC) for working memory (WM) function, although this system has rarely been studied in humans in vivo. However, dopamine and PFC activity can be directly measured with positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), respectively. In this study, we examined WM capacity, dopamine, and PFC function in healthy older participants in order to test the hypothesis that there is a relationship between these 3 factors. We used the PET tracer 6-[18F]fluoro-L-m-tyrosine to measure dopamine synthesis capacity in the striatum (caudate, putamen), and event-related fMRI to measure brain activation during different epochs (cue, delay, probe) of a WM task. Caudate (but not putamen) dopamine correlated positively with WM capacity, whereas putamen (but not caudate) dopamine correlated positively with motor speed. In addition, delay-related fMRI activation in a left inferior prefrontal region was related to both caudate dopamine and task accuracy, suggesting that this may be a critical site for the integration of WM maintenance processes. These results provide new evidence that striatal dopaminergic function is related to PFC-dependent functions, particularly brain activation and behavioral performance during WM tasks.

Dissociation of metabolic and neurovascular responses to levodopa in the treatment of Parkinson's disease

We compared the metabolic and neurovascular effects of levodopa (LD) therapy for Parkinson's disease (PD). Eleven PD patients were scanned with both [15O]-H2O and [18F]-fluorodeoxyglucose positron emission tomography in the unmedicated state and during intravenous LD infusion. Images were used to quantify LD-mediated changes in the expression of motor- and cognition-related PD covariance patterns in scans of cerebral blood flow (CBF) and cerebral metabolic rate for glucose (CMR). These changes in network activity were compared with those occurring during subthalamic nucleus (STN) deep brain stimulation (DBS), and those observed in a test-retest PD control group. Separate voxel-based searches were conducted to identify individual regions with dissociated treatment-mediated changes in local cerebral blood flow and metabolism. We found a significant dissociation between CBF and CMR in the modulation of the PD motor-related network by LD treatment (p < 0.001). This dissociation was characterized by reductions in network activity in the CMR scans (p < 0.003) occurring concurrently with increases in the CBF scans (p < 0.01). Flow-metabolism dissociation was also evident at the regional level, with LD-mediated reductions in CMR and increases in CBF in the putamen/globus pallidus, dorsal midbrain/pons, STN, and ventral thalamus. CBF responses to LD in the putamen and pons were relatively greater in patients exhibiting drug-induced dyskinesia. In contrast, flow-metabolism dissociation was not present in the STN DBS treatment group or in the PD control group. These findings suggest that flow-metabolism dissociation is a distinctive feature of LD treatment. This phenomenon may be especially pronounced in patients with LD-induced dyskinesia.

Metabolic abnormalities associated with mild cognitive impairment in Parkinson disease

OBJECTIVE

To use (18)F-fluorodeoxyglucose (FDG) and PET to investigate changes in regional metabolism associated with mild cognitive impairment (MCI) in Parkinson disease (PD). Cognitive abnormalities are common in PD. However, little is known about the functional abnormalities that underlie the manifestations of MCI in this disorder.

METHODS

We used FDG PET to measure regional glucose metabolism in patients with PD with multiple-domain MCI (MD-MCI; n = 18), with single-domain MCI (SD-MCI; n = 15), and without MCI (N-MCI; n = 18). These patients were matched for age, education, disease duration, and motor disability. Maps of regional metabolism in the three groups were compared using statistical parametric mapping (SPM). We also computed the expression of a previously validated cognition-related spatial covariance pattern (PDCP) in the patient groups and in an age-matched healthy control cohort (n = 15). PDCP expression was compared across groups using analysis of variance.

RESULTS

SPM revealed decreased prefrontal and parietal metabolism (p < 0.001) in MD-MCI relative to N-MCI, as well as an increase in brainstem/cerebellar metabolism (p < 0.001) in this group. In these regions, SD-MCI occupied an intermediate position between the two other groups. PDCP expression was abnormally elevated in the N-, SD-, and MD-MCI groups (p < 0.05), increasing stepwise with worsening cognitive impairment (p < 0.01).

CONCLUSIONS

Early cognitive decline in Parkinson disease as defined by mild cognitive impairment is associated with discrete regional changes and abnormal metabolic network activity. The quantification of these alterations with (18)F-fluorodeoxyglucose PET may allow for the objective assessment of the progression and treatment of this disease manifestation.

Metabolic correlates of subthalamic nucleus activity in Parkinson's disease

Overactivity of subthalamic nucleus (STN) neurons is a consistent feature of Parkinson's disease (PD) and is a target of therapy for this disorder. However, the relationship of STN firing rate to regional brain function is not known. We scanned 17 PD patients with (18)F-fluorodeoxyglucose (FDG) PET to measure resting glucose metabolism before the implantation of STN deep brain stimulation electrodes. Spontaneous STN firing rates were recorded during surgery and correlated with preoperative regional glucose metabolism on a voxel-by-voxel basis. We also examined the relationship between firing rate and the activity of metabolic brain networks associated with the motor and cognitive manifestations of the disease. Mean firing rates were 47.2 +/- 6.1 and 48.7 +/- 8.5 Hz for the left and right hemispheres, respectively. These measures correlated (P < 0.007) with glucose metabolism in the putamen and globus pallidus, which receive projections from this structure. Significant correlations (P < 0.0005) were also evident in the primary motor (BA4) and dorsolateral prefrontal (BA46/10) cortical areas. The activity of both the motor (P < 0.0001) and the cognitive (P < 0.006) PD-related metabolic networks was elevated in these patients. STN firing rates correlated with the activity of the former (P < 0.007) but not the latter network (P = 0.39). The findings suggest that the functional pathways associated with motor disability in PD are linked to the STN firing rate. These pathways are likely to mediate the clinical benefit that is seen following targeted STN interventions for this disease.